I recall reading about an impedence matching technique where the conductors
of, say a coax segment, would be spaced slowly more and more apart (like the
braid becoming larger like the shape of a funnel) and extending until only
one conductor (the center conductor) is required for transmission. A similar
setup at the other end of the run would transform the impedence back to
usable levels.
The drawings in the article portrayed the transformation as appearing
like a funnel flaring out slowly with the center conductor eventually the
only conductor - then the same setup at the other end.
The idea of a single-conductor transmission line makes it an inviting
idea, but physically building the impedence matching sections on the ends
look like a real challenge.